Boat Stability help pls.

Discussion in 'Boat Design' started by trouty, Oct 27, 2005.

  1. trouty

    trouty Guest

    I have a few (OK a LOT) :D of questions I could do with some help on please.

    I have a 25 ft alloy vessel in commercial 2C survey.

    It has a stability booklet with it.

    It was built to carry 4 govt officers, ie currently has 'provided seating' within the pilot house style cabin for 4 people (2 passengers & 2 crew).

    I intend to use the vessel, for '6 pack' type charter work.

    I would LIKE to increase the surveyed passenger carrying capacity by 2 more to 6 in total if possible (4 + 2) Survey 2 C out to 30 Miles.

    She has full flotation foam (Glass encasulated) under the main deck and ALL 2C safety gear.

    She has 320 litres fuel under the deck in two transvers tanks just aft of the pilot house cabin.

    She has twin 115 Honda 4 strokes on the rear end.

    She carries 4 (2 banks of 2) Marine batterys in two battery boxes, 2 starb & 2 port immediatetley aft of the cabin.

    I've been reading as much as I can about the stability discussions with regard to the ethan allen tragedy etc. I did pass "stability & ships construction" during my skippers ticket study / exams. i.e.- I have a basic understanding of vessel stability, metacentric height, righting levers & roll period etc...I've read quite a bit on it but don't claim to be a naval architect or understand all the nuances of the proceedures.. I'm not toally dumb - but possibly nopt far from it in terms of what a naval architect would understand in this regard.

    I have very little idea really - what all the areas under the graph line really mean in the stability booklet provided with the vessel.

    What I do know - is that just about EVERY 25 ft similar V hull surveyed vessel I've seen of this length/type (25 ft length, 8 ft 4 inch beam) have carrying capacities - well above 4 people - some as high as 8 + 2, some at 6 + 2, this includes surveyed hire drive vessels etc and those in 2C for fishing and diving charters. I've seen one at 10+2 and no it wasn't a cat hull with good block waterline stability...I'd swear it was pretty similar to mine.

    I can show links to where photo's of these vessels are on the net with thier for sale adverts and details of their surveyed carrying capacity's!

    I suspect that my vessel was originally 'surveyed' by the states surveying authority to carry only 4 in total, because they built it for that purpose!! (for themselves). (The vessel was commisioned / built for the marine safety branch of Dept Transport in yr 2000 and was a patrol / rescue vessel).

    One wonders how they would haul anyone aboard in a rescue situation, if the darn thing was already almost at "capacity" with it's crew of 2 (or at capacity if 4 officers were aboard - depending on the day and excercise it was performing)? Do they pass on bye a sunk boat with people in the water in a "rescue boat" because it's already full up with crew???????:rolleyes:

    Anyway - back to the problem - how do I determine if the boat can safely carry more than it's current Survey of 4 people? (And then how do I convince the surveying authority of this)?

    I see that Someone (Tom) posted this formulae in the ethan allen thread.

    Simplified Stability test 46 cfr 178.330

    Mp = (W) (Bp)/6

    Mp = Passenger heeling moment in kilogram meters ('s)

    W = Total passenger weight using 72.6 Kg's (160 pounds) per passenger or for smooth water (E survey) 63.5 Kg (140 pounds) per passenger.


    Mw = (P) (A) (H).

    Can someone please explain for me what (Bp) stands for in the above equation? (And if it's value will be included in my stability book, what it might be called so i can locate it?) Similarly with (P) & (A) & (H)?

    How do I determine passenger heeling moment as a layman?

    Can it be done tied up in smooth water at the marina, tying a plumb bob on a length of string off the stern light fitting in the centre of the rear of the cabin roof, and marking centre line on the deck with a pencil - at rest with no passengers (and me standing on the centreline to mark the centre position), Then add various numbers of passengers all at the same side & marking (measuring / recording) the distance the plumb bob deviates at different numbers of passengers as she heels? If the height of cabin roof above deck and plumb bob deviation distances are known - can passenger heeling moment be easily determined?

    The boat has a self draining deck, (from memory about 150 mm / 6 inches) above waterline at rest, which drains thru two spring loaded scuppers at the stern. It also has a stern access door thru the transom, (A proper one - full width ally, same dimensions as transom and hinged / locked with appropriate spring loaded bolt..above scupper line.) Does height of the self draining deck above waterline at heel have a bearing upon survey capacity? (Or is if the standard 1/8 freeboard inclination test (to topside of gunnel?)

    Also someone posted that under ABYC for boats under 26 ft (Remember mines 7.5 meters or 25Ft) as of 07/04 that:-


    Mw capacity is determined by attaining boats cubic capacity below static float plane(Defined as something similar to the margin line) Converting this valueto the weight of water it would displace, subtracting the boats weight and permitting one poundof load capacity for every seven pounds of remaing displacement.

    The number of persons shall not exceed the value determined by dividing the quantity of 32 the posted persons pound capacityby 141 and rounding up or down to the nearest whole number.

    Yes - I know maybe some of you will just say that I should stick with what it's rated / surveyed capacity already is, facts are -- vessels that carry more people are more viable. That said - I certainly don't want to be the skipper of a vessel - like the ethan allen where ANYONES life is unecessarily lost. That why I am asking questions / studying the detail - and trying to understand IF indeed ANY spare capacity is safely there for extra passenger carrying capacity, and if so how much?

    Heck - theres 5 in my own family, I can't even legally take us all out for a days fishing / cruising!). Someone has to stay home - & that goes down like a lead ballon with she who must be obeyed!!!

    I'm trying to be 'responsible' about this - and do my homework FIRST.


    More detail bout the vessel.

    The damn thing was built for the Transport Marine safety authorities, by a local boat builder. He got a naval architect (Who's now VERY long in the tooth and a little bit shall we say "doddery" due to advancing years), to calculate the stability booklet 5 years ago, when he was a little more in control of his mental faculties than today.

    I did contact this fellow (no names no packdrill - he's old and it can happen to any of us, - he's earned his retirement). He could hardly remember determining the stability booklet for this vessel, or what capaicty it had or now has or could have - heck - I doubt he could remmber his own name that week...maybe it was a bad week for him. (I'm half his age and I have trouble remembering my own kids names!).

    Anyway - he's retired, he's old and he can't help me (Not that he isn't still more than willing to have a go...I would fell guilty even asking him). He did remember that the vessel had difficulty passing it's original stability test and that he had the builder add sponsons to the hull at the chine - in order to get it to pass stability. (More on this in a minute).

    I've asked another local naval Archtect to take a peek at the stability booklet, but in fareness - although he's a close friend - he's also 'retired', but always very very busy (designing boats go figure, some retirement!). ;)

    I have no doubt in time he will figure it all out for me - but to be truthfull - I actually WANT to understand this myself..properly. I believe as a skipper - I have a responsibility to my passengers & crew do so.

    Thats why I'm here asking questions.

    I'll try and link in a photo of the vessel below this post in a moment - to give you an idea of what it looks like.

    I don't think I can scan in the stability graphs (or understand what all the wind heeling moments etc all mean). I could fax them to someone - if they have the ability to scan them in and post them here (or can understand / interpret them)?

    With regard to the sponsons....

    The darn things do add stability at rest, heck I've had 4 of us (and I weigh 120 kilos') + 6 scuba cyclinders aboard, all of us on one side at one time - and the darn thing hardly lists at all - it IS a very stable boat - I already KNOW this much..I can tell it's capable of carrying MORE than what she is currently surveyed for I think or I wouldn't be contemplating trying to increase her surveyed capacity. I just "feel" she has more capacity than her current certificate would initially indicate...but I could be wrong!

    The one drawback I've found so far (which I was alerted to by the previous marine safety officer who used to skipper her) is an unfortunate trend, if you trim her incorrectly (she has Bennet trim tabs and prefers a little nose up attitude with OB trim & tabs to keep the leading edge of the sponsiosns from burying), to bury the leading edge of one of the sponsons and to corckscrew into quite an alarming heel at speed if you turn broadside to swell - with the nose at all trimmed down. If your not expecting it and haven't experienced it - it is quite disconcerting first time - but she breaks out and heels back before threateniong to tip over in any way, but the excessive heel angle does make the inexperienced squawk a bit first time....I know I left skidmarks in the undies first time - even tho I'd been warned in advance!

    I am trying to do the right thing here....I've probably included erroneous info - and NOT included the info you need to help me so feel free to ask me questions - if it's info I can extract from the stability booklet to help answer the questions I've asked.

    Can someone help me to understand this properly please?:confused:

    Many thanks in advance.

  2. trouty

    trouty Guest

    Ok, Da Photo


    Hope this works, so you can see the sponsons.

    Lets not revisit the trailer hitch height issue again, she now has a load leveller device and rides nice n level - and the vehicle is licensed to haul the 3500 kilos all up weight of boat & trailer, and she has the full vaccum over ride breaks system etc even if she's not the 7.1 litre turbo diesel 4 door ford powerstroke F 250 I'd really like.:)

    Many thanks again!

    P.s - can anyone edit this darn photo smaller pls so it don't blow the post width out like this, board moderator - please feel free to do so..I hate chewing up your bandwidth like this...but my capacity on win 95 & 28800 bps is very limited to do much more than you see here...sorry.
  3. cyclops
    Joined: Feb 2005
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    Location: usa

    cyclops Senior Member

    I would make a appointment with whatever marine group passes final and LEGAL judgement on all "6 pack" requirements. Tell them you can send photos to see if there is any hope. They like to be able to say it is a possibility. Rather then walk up to a hopeless application. Good luck.
  4. BrettM
    Joined: Apr 2002
    Posts: 204
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    Location: Australia

    BrettM Senior Member


    Firstly, this is an Australian boat under Australian Survey.

    You should be looking at the USL code Section 8 for the required criteria. It should be written in the front of your booklet or your NA friend should have it if you are interested.

    I agree that most vessels that are this size normally carry more than 4 persons. As it happens, 12 is the maximum passengers allowed under 2C subject to passing stability requirements of course.

    2C vessels less than 16m length are subject to a minium GM and deck edge immersion criteria. There are however, options for a more detailed stability analysis using righting arms etc. if your vessel dowsn't fit neatly into the criteria.

    In reality, your best advice is to contact a local practising naval architect or the WA authority (I believe is where you are) and discuss it with them.

    You will be required to submit a new stability booklet (or addendum to the current one)with calculations to show compliance with the relevent criteria.

    You may also be required peform a new inclining experiment (sounds like one was done previously) since it was several years ago since it was done. Talk to the authorities to find out what needs to be done.

    Safety equipment etc will also need to be certified for the increased numbers if you haven't already.

    Just a quick answer here really, as each boat is different. Anyway, I hope this helps. Now go pick up the phone.
  5. trouty

    trouty Guest

    Thanks Cyclops & Brett,

    I have no problem at all - with engaging a local practicing Naval architect to do the papaerwork and incline tests etc - and to deal with the WA surveying authority (at least they speak the same language!). (I already have someone working on it!).

    Thing is, I actually do want to understand this - it can't be rocket science!

    I want to understand - what the graphs in my stability booklet mean, and I want to have a pretty good idea as skipper - just what load she will carry and where - because - you never know - there could come a day - when I have to take on extra passengers from a vessle in distress - and the propper knowledge of how many is too damn many and at best - where I need them to all stand / sit so as to retain maximum stability in bad conditions - is all -valuable knowledge to me.

    I guess I could ask the question - whats the point of including a stability booklet with a vessel, if the skipper can't make head or tail of it?.

    The booklet for example, at the beginning says:-
    This is just what I'm attempting to do....

    Sadly the rest of it is pretty much me at least..

    For example - the table of weight movements & pendulum readings...

    Sure all the figures are there, and a plan of the position and weights used for the inclining experiment etc....but no "comment" to say what it all means...for the operator...I do know for example - that there were 6 readings taken, that the pendulum length was 1.645 meters, and the table total was 15.72 with an average of 2.620

    But - What does that mean?

    Freeboard measured from sheer at the bow 1.250 M
    Height from Baseline at that position 1.630M
    Draught fwd 0.380M
    Freeboard measured from the sheer at stern 0.840M
    Deck from sheer at that position: 0.690M
    Freeboard from deck aft 0.150M
    Deck Fram Baseline at that position 0.590M
    Draft aft 0.440M

    Draft Fwd 0.380
    Draft Aft 0.440
    Mean Draught 0.410M
    Trim 0.60M

    Calcs to determine the "as inclined" condition:
    From table of weight movements & Pendulum readings"

    Average for W x d / M = 2.620
    Gf Mo = W x d x Pend Length / Disp = 2.620 x 1.645M/3.10 = 1.3903M

    and KG = KMT - Gf Mo = 2.016 - 1.39 = 0.626M

    From page "AS inclined" - we find LCB = LCG = +0.655M from designated midship position.

    Please Note: The program used for conditions of loading requires the App as the datum for input of LCG

    A positive value for LCB means this distance is abaft Midship.

    Therefore the distance from App to LCG is 3.09 - 0.655 = 2.435M

    From here on - all the hydrostatic data tables are reproduced - and to me they don't mean diddly squat...

    There are 7 pages of these computer generated graphs and tables...which I am sure mean something meaningfull - but no explanatory notes...for the layman operator!!

    Then theres a page for calculations of wind heeling lever, and a formulae and a neat table of numbers...which I guess tells me - the harder the wind blows - the more she heels - right? ;o)

    Then theres 3 more pages of computer hydrostatic tables...abd basically thats it.

    The "general comments" says:-
    Other than all the tables and graphs - I'm pretty much on my own.

    Now - if I only knew what them numbers 'n pictures all mean????

  6. trouty

    trouty Guest

    More info.

    I went looking Brett for the stability criteria (presumeably under USL code section 80 you mentioned listed in the front of the booklet...

    This is what it says:-

    Now - I'm assuming all those tables....of hydrostatic values...relate to how well - the vessel in it's inclining experiment, - meet the perameters of the stability criteria??

    err = whats a radian? and do I need a lead suit not to get radiation poisoning from it?:confused: :D

    Err - cheers I think!
  7. jam007
    Joined: Sep 2005
    Posts: 89
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    Location: Sweden

    jam007 Junior Member

    Not radiation poisoning, maybe Math poisoning. :)
    The circumference of a circle is 2*pi*radius. A radian is the angle of the circle sector ( A piece of a round cake) that you get if you put the lenght of the radius on the circumference of the circle.
    In other words 2*pi radians is 360 degrees. One radian is approx. 57 deegrees
    Radians make math much simpler but takes a while to get used to...

    Anders M
  8. trouty

    trouty Guest

    Hmmm pie!!!!

    ;) The Homer Simpson in me says...Hmmmm Sector of caaaake, Hmmmm Pieeeee...

    I think i could get to like this boating maths - next you'll be tellin me theres Hmmm Doughnuttttss!!!:D

    Thanks for that - radians I think I can come to grips with...sounds simple enough...I like simple if you haven't gathered! :)

  9. Stephen Ditmore
    Joined: Jun 2001
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    Location: Smithtown, New York, USA

    Stephen Ditmore Senior Member

    Latest Proposed Australian Regs

    The latest proposed Australian regs are available for public comment until January 10. Go to, click on "Have Your Say," then "Documents for Comment," then "Draft Document C6B." Go to Chapter 5 and use the formulae to calculate heeling moments.

    Then go to "Draft Document C6A" Chapter 6, see how much freeboard loss your boat is allowed, and then try it. You'll need some 55 gallon drums and a water pump with hose with which you can fill the drums in a reasonable amount of time. 1 cubic meter of fresh water weighs 1000kg. Salt water 1025kg. I'm not sure how many gallons are in a cubic meter...

    Alternatively, recruit some friends and have them stand where you tell them, or use cinder blocks. This is not instead of having a professional do it, it's just so you'll have an idea where you stand before you start writing checks.

    If the draft rules are not clear, this is your opportunity to tell the rulemakers what needs fixing!
  10. trouty

    trouty Guest

    Thanks SD

    A very interesting link, (The pdf pages are still downloading) - I'll read em at length and try n make hear or tail of them.

    Probably what will interest me most - is whether the draft rules are still so archaic as to prevent some of themost high tech, "flats boats" from the US - ala Hews Maverick etc from attaining a survey cert downunder, thus preventing any fly fishing charter tours in anything bar the ark, whhilst in the USA - 6 pack tours in such craft are de rigour!

    I'll do my best to make comment, if I find that to still be the case - usually the deliberate design lack of freeboard to improve windage for poling makes such craft unable to pass the loss of freeboard incline tests here for paying passenger survey... (thus meaning the fly fishing charter industry here basically doesn't (can't legally) exist - despite having world class waters suited to such and a tourism industry clamouring for such activities - go figure!) hence interesting to see if that would still be the case in the new draft rules!.

    Stupidly - any fool (e.g. any international angling tourist unfamiliar with our waters) can still hire (rent) or buy any boat without any survey for recreational use, without any form of license to operate....and go fly fish wherever he or she wants - whether it's safe to do or not - but those who want to provide that service, legally and abide bye the rules - for public liability and hence insurance purposes - literally can't do so..

    We are a bass-ackwards bunch...lets see if these new rules change anything! I'm willing to bet they probably don't!

    It wouldn't surprise me in the least!

  11. BrettM
    Joined: Apr 2002
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    Location: Australia

    BrettM Senior Member


    Its probably too complicated to explain here, but perhaps you need a book that will help you. I have a book called "Teach yourself Naval Architecture" which was given to me (as a joke) that turns out is quite handy for referenceing the simple things and details how inclining experiments work etc. Not sure that it is still in print.

    FYI and very quickly,

    The hyrdostatic tables refer to the properties of the hull at various drafts. EG at X draft and zero trim, the displacement would be Y. etc etc.

    The inclining experiment is purely to determine the locations of the centre of gravity in the inclinined condition. The displacement & longitudinal CG are determined by taking freeboard measurements, converting them to drafts (with trim) and relating them back to the hyrdostatic tables to determine. LCG is always directly above the centre of buoyancy so LCG=LCB from the tables.

    The pendulum movements and calculations determine the VCG of the vessel. I am not going to go through the math here, just steer you in the right direction. Note that the reduction in effective VCG due to the free surface of any tanks is accounted for here also.

    A lightship derivation is then performed calculating the boats displ. VCG and LCG without fuel, water, people or inclining weights and equipment. This value is the starting point for the loading conditions you will see in the booklet.

    The graph you speak of is the GZ of righting arm curve. At each value of heel (x-axis) the righting lever is calculated forming a cuve that starts at zero and increases before returning to zero (angle of vanishing stability - capsize) ant some other angle. If you multiply the GZ at any angle by the displacement you get the righting moment or to the layman, the "torque" response of the boat. The higher the value, the more the boat wants to return to the upright. Your graphs should also be terminated at an angle of downflooding which is the point at which the hull openings enter the water and the vessel is flooded and sunk.

    As it turns out, the area under the curve is a measure of the engergy required to upset the boat, hence the minimum requirements (metre-degree or metre -rad) which are set by the authorities.

    The heeling levers which are also shown on the graph are the upsetting moments/displacment.
    EG X persons moved Y metres is a upsetting moment. Divide this by the displacement and you get a curve that can be transfered to the GZ curve. Where the line meets the GZ curve is the equilibrium position. Note that the heeling (upsetting levers) are normally reduced by a factor of cos(angle).

    there you go - Rocket science, I'll send you my bill. :) I hope that this helps somewhat, pretty basic explanation, so I suggest you do some further reading.


  12. trouty

    trouty Guest

    Thanks Heaps Brett

    It's a bill I'd gladly pay because you make a very complex subject - simple enough in laymans terms for me to get an understanding - and thats indeed what I'm looking for.

    I am doing some serious reading trust me.

    So far after about 4 hours downloading last night (which got me in strife with she who must be obeyed) - I have section 6 of the new draft survey rules for Oz - and today I'm going to aim to get section 5.

    I'm only on 28800 bps - so doing this stuff takes me ages - coz I'm tying up the household phone line etc..and well - you know how it is.

    I'm trying to read all these 76 pages of Pdf documents and get some understanding - but having the 76 pages "condensed into a simple meaningful explanation the way you have certainly makes the larger document more meaningfull to me. (I can go back to your explanation when I get lost inside some of the documents detailed formulae etc..)

    I'm determined to crack this nut and am determined to conduct some of my own inclining experiements on my own vessel - just for the sheer sake of doing so...

    I have an ulterior motive.

    I'm taeching my son - to be as smart as his ol man (he will eventually skipper the vessel....he's 18 years old and already half way to his skippers ticket), andI want HIM to understand this stuff, - which means I have to understand it first - to be able to teach him.

    Must be the masochist in me but I enjoy learning new stuff and the internet is like having access to all the worlds university's and some of the worlds smartest people as teachers - so I'm unashamedly making the most of it.

    I sincerely thank you for actually answering the question I basically asked (in my own dumb round about way).

    I'll report back when I've learned something.

    Cheers & thanks again - I'm gonna save and print your reply and pin it up above the 'puter here to study while I'm trying to fathom my stability booklet..I'm determined to master this.
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